We've had this debate many times. Historically MD was southern, and many residents still use that historical reference. If you look at measures such as membership rates in non-black evangelical churches or listeners to country music stations, MD doesn't look very southern anymore.
I think those that claim that Maryland isn't "purely" southern have a point, but few make a good case for it being Northeastern.
It is 29% African American. No non-Southern state is more than 20%.
It doesn't have the "white ethnic" populations of the Northeast. Where are the Italians and Irish? How much of the Hispanic population is Puerto Rican?
It also has a lot of unincorporated areas rather than towns/boroughs and the suburbs of DC look more Sunbeltish and "boomburb"-ish than than they look like the "quaint suburban towns and old industrial cities" model that prevails in most of the Northeast.
Of course, being in the national capital region it's going to have a lot of transplants and be an outlier culturally and politically.
Demographically, it resembles Virginia more than it does any other state. Maryland is a border Southern state.
This may seem like a small thing, but take a look at the sort of housing stock in various major cities across the USA. Philadelphia- and especially the poorer parts of Philly- has a distinctive rowhome style which you don't really see in NYC or points north, or out in the Midwest or South either.
But you do see it, in spades, in Baltimore.
Perhaps evidence for a shared Mid-Atlantic identity somewhere in there?
I was going to make a similar point that Balto shares much in common with Philly. The architecture and history follow very similar paths. I would take train as the authority, but rowhouses in parts of Queens and Brooklyn remind me of their counterparts in Balto and Philly. In my visits I find more similarities than differences.
As to the demographics, Balto was a significant some to white ethnic immigration and was the second largest port of entry after NYC. Residents of Irish ancestry make up 13% of the population in Balto. Little Italy is an important neighborhood and was the childhood home of Nancy Pelosi, the daughter of Baltimore Mayor D'Alesandro. Before the Civil War the black population of Balto was predominantly free, though there were some slaves, and today makes up the largest ethnic group as it does in Philly.
